A passive imaging in which an image is received in real time by using a millimeter-wave is able to obtain the image of all objects that include building and human body without being influenced by the weather, because of this the commercialization is being expected. The millimeter-wave indicates the electromagnetic wave in which the wave-length is approximately the range from 10 mm to 1 mm, and corresponds to 30 GHz to 300 GHz as the frequencies. In case of comparing it with ones of microwave band, the electromagnetic wave of millimeter-wave band has the characteristics such as: a) a small and light system can be realized; b) the interference and radio interference can be hardly caused because the narrow directivity is obtained; c) information of large capacity can be treated because the frequency band is wide; and d) a high resolution can be obtained when it is used to the sensing, and also has the characteristics such as: e) the attenuation due to fog or rain is very small; and f) the transmissivity to dust or small dust is good and it is strong for environmental conditions, In case of comparing it with ones of visibility or infrared range.
In an imaging system which uses the millimeter-wave, there are two methods of an active imaging and passive imaging if it is roughly classified. The active imaging is the one that irradiates to the object the coherent millimeter-wave radiated from an oscillator and receives and detects the reflective wave or transmissive wave and obtains the image corresponding to the received strength or phase. This method is used for a radar and plasma electron density measurements etc.
Also, the passive imaging is the method which receives widely the millimeter-wave portion in the thermal noise that every object is radiating in proportion to the absolute temperature and detects and amplifies this and obtains the image. Although there are advantages such as: it does not require the oscillator; and also there is no influence of the interference in order to receive the coherent wave and the signal processing is easy, a receiver with the low noise and high sensitivity is required because the receiving-signal is the very feeble one that is the thermal noise. This method is used for a radiometer that measures the ozone and carbon monoxide etc. in the atmosphere, and for the field of radio astronomy etc.
This real-time passive imaging which uses the millimeter-wave is performed by receiving the thermal noise generated from the objects 100 such as a human and thing etc., by a receiving element for imaging 102 that was arranged at a focal position of a lens antenna 101 through the lens antenna that has a circular directivity, as shown in FIG. 21. Because of this, the development of the receiving element (antenna) for imaging that matches the lens antenna 101 has become extremely important. Usually, a diameter (D) of the lens antenna 101 is designed to be equal to the focal distance (f), and it is assumed that the passive imaging of best condition is performed when an f/D is equal to 1 (here, f/D means: f is divided by D).
Although there is a method in which a mechanical scanning is used in the real-time imaging method, a complex mechanism for scanning is required in this method and also it takes a lot of time for measurement, therefore it is difficult to obtain the real-time image. On the other hand, an imaging array method in which many receiving elements are arranged in two-dimensions and the image is obtained does not require the scanning mechanism and is able to measure it in short time, thereby being able to perform the real-time imaging. In FIG. 21, though one receiving element for imaging 102 is illustrated, a plurality of receiving elements for imaging is being arranged side by side in the array shape, actually.
Further, as antenna which is suitable for this receiving element for imaging 102, because the lens antenna 101 has the circular directivity it is required that the directivity of E-plane and the directivity of H-plane are almost equal in order to match this lens antenna 101. Here, the E-plane (x-z plane) is a resonant plane of the electric field and the H-plane (x-y plane) is a plane perpendicular to the E-plane. Generally, even if it strongly resonates for the E-plane and can receive the image from the object, there are many cases in which there is no directivity of the H-plane, so there are problems such as: the conversion efficiency decreases and a gain becomes low too.
Also, as for the characteristics which are required further, other than the one which is a broadband and which is suitable for integration and array, it is desired that as many antennas as possible in a specified area etc. can be arranged because the number of array elements determines the pixels of imaging. Furthermore, it needs to amplify a received signal until the noise level of a detector, so it is required that the antenna has a high gain in the meaning of decreasing a loss to an amplifier.
As a dominant antenna that satisfies these requirements, the research of a TSA (Tapered Slot Antenna) is being carried out prosperously, recently. This TSA is a broadband, light-weight and thin-shape, and is able to be made easily by the photolithographic technology and is integrated easily, so it is being used for various kinds of usage such as the communication-use and measurement-use from the frequency band of the microwave to the millimeter-wave. A fundamental principle of operation of this TSA is explained as a traveling-wave antenna. In other words, it is different from a reflective-type antenna such as a dipole antenna, and it is being understood as the antenna by which a generated electromagnetic wave is propagated to the traveling-direction without vibrating as it is. Then, as taper shapes of the TSA, a Linear-TSA and a Vivaldi-TSA (that is a taper shape with an exponential function of trumpet-type) are used well.
Also, a CWSA (Constant Width Slot Antenna) in which several different function forms were connected and a BLTSA (Broken Linearly TSA) which has the taper shape in which the LTSA was bent and connected are proposed.
Furthers a tapered slot antenna TSA called a Fermi-antenna is also being proposed recently, and a structure of this Fermi-antenna 10 has a taper shape that is represented by a Fermi-Dirac function (called “Fermi function”, hereinafter), as shown in FIG. 22, and also has a corrugation structure 12 of comb shape in the outside of a dielectric substrate 11. This Fermi-antenna 10 is being considered to be suitable for the receiving antenna for millimeter-wave imaging, because the facts in which the directivities of the E-plane and H-plane are almost equal even though the width of the substrate is narrow and also the levels of side-lobes are comparatively small are being found out experimentally.
FIG. 22 is the one showing a fundamental structure of the Fermi-antenna 10, and the characteristics of this antenna are to have the taper shape which is represented by the Fermi-Dirac function and the corrugation structure 12 in the outside of the dielectric substrate 11. This Fermi-antenna is advantageous in the following points; it can be easily made on the dielectric substrate 11 by using the photolithographic technology and, the antenna and feeding circuits can be configured on one side of the dielectric substrate 11. The Fermi-function is the one that is known as the function that represents the energy-level of electron in the quantum mechanics, and it generally becomes a function that is given by the “equation 1”, when the structure and coordinate system of FIG. 22 are considered.
                              f          ⁡                      (            x            )                          =                  a                      1            +                          ⅇ                              -                                  b                  ⁡                                      (                                          x                      -                      c                                        )                                                                                                          {                  Equation          ⁢                                          ⁢          1                }            
Here, a, b and c are the parameters that represent the taper shape. The “a” represents an asymptotical value of the function when X approaches the infinity, and the “c” is a point of infection of the function. Also, from f′ (c)=ab/4, the “b” is a parameter that determines tangential gradient at the point of infection. Here, if there are the relations of the f(c)=a/2 and also b(L−c)>>1, the X=L is assumable at near of an aperture, and it becomes f(L)=a, consequently the width of the aperture W is given by W=2a. In addition, as the design parameters of Fermi-antenna, a relative dielectric constant ∈r of the dielectric substrate, the thickness of the substrate h, the length of antenna L, the width of corrugation structure Wc, the pitch p, the height of corrugation Lc and the Fermi-functional parameters a, b and c that determine the taper shape are extremely many, therefore how these values are chosen if the antenna that is small and that has the circular directivity of desired beam width BWdesign can be designed has become an important subject.
With respect to this Fermi-antenna, the paper which showed that the side-lobes of H-plane of the Fermi function tapered TSA are reduced most in comparison with the LTSA, Vivaldi, CWA and BLTSA and the TSA which uses the taper with Fermi function at 60 GHz frequency is proposed (for example, referrer to the published document 1). In this published document, it is shown that though the directivities of E-plane and H-plane will differ when the width of substrate of the Fermi-antenna becomes narrow, the directivities can be made almost equal by providing this the corrugation structure.
Also, the inventors obtained a radiation directivity by a FDTD (Finite Difference Time Domain) method when the taper shape of Fermi-antenna (namely, Fermi functional parameters; a, b, c), the length of antenna L, the thickness of dielectric h, the aperture width W and the width of substrate D were changed, and did clarify the relationship between the various parameters relating to the structure of antenna and the characteristics of antenna, and proposed an optimal structure of the Fermi-antenna that was suitable for the receiving element for imaging (referrer to the published document 2). FIG. 23 is the one showing an example of the measures of typical Fermi-antenna that were proposed here. According to this published document 2, an operating gain was 13.2 dBi (here, “i” means “isotropic”) and the levels of side-lobes of E-plane and H-plane were −18.4 dBi and −14.3 dBi respectively, and also it had a good axis symmetry and it was reported that the result which accorded well with an experiment was obtained, in the Fermi-antenna with the width of substrate D=0.58λ0 and aperture width W=0.32λ0. In this example, the measures of typical Fermi-antenna that were designed at 35 GHz are shown, and these are c=2λ0=17.14 mm, a=W/2=3.9 mm and b=0.28 mm−1, here.
However, the TSA that includes a Fermi-antenna has many structural parameters such as a function that determines the taper shape, the length of antenna, the aperture width, the finite width of substrate and a relative dielectric constant, and has a characteristic that the radiation characteristic changes largely in accordance with the changes of these. Because of this, there were no method other than an empirical method according to the experiment and a method according to the approximate computation when the Fermi-antenna was designed. In other words, in the present, even if the TSA was made and the one having a good characteristic was yielded by chance, the characteristic has changed whenever it was made, and therefore it was the situation in which a firm design theory was not being established. Like this, there is such reality that is not easy to obtain the design guideline that realizes the radiation directivity required to the Fermi-antenna, and the design method of the TSA having a circular directivity was not presented even in the proposal described in the above-mentioned published document 1 and published document 2.
[Non-patent Document 1] S. Sugawara etc. “An m-m wave tapered slot antenna with improved radiation pattern”, IEEE MTT-S International Microwave Symposium Digest, pp. 959-962, Denver, USA, 1997
[Non-patent Document 2] The Institute of Electronics, Information and communication Engineers transactions B. Vol. J80-B, No. 9 (2003.9)